qemu: Fix starting of VMs with empty CDROM drives

Since 24e5cafb (thankfully unreleased)
when a VM with an empty disk drive would be started the code would call
stat() on NULL path as a check was missing from the callback rendering
machines unstartable.

Report success when the path is empty (denoting an empty drive).

src/qemu/qemu_driver.c

@@ -343,6 +343,9 @@ qemuSecurityChownCallback(virStorageSourcePtr src,
     if (virStorageSourceIsLocalStorage(src)) {
         /* use direct chmod for local files so that the file doesn't
          * need to be initialized */
+        if (!src->path)
+            return 0;
+
         if (stat(src->path, &sb) >= 0) {
             if (sb.st_uid == uid &&
                 sb.st_gid == gid) {